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Sep 12, 2026 - Detecting Faults on the National Grid



How does a 5000 amp transmission circuit get measured safely by a protection relay? Or a 63,000 amp fault?
In this video, we take a deep dive into "HV AC current transformers (CTs)": what they are, how they work, why they`re critical to grid protection, what the ratings on their nameplates actually mean, the different types you`ll find on the electricity network, and how we test them during commissioning.
We`ll cover:
• How CTs transform thousands of amps into a small, usable secondary current
• CT ratios, polarity and why getting them backwards can cause protection to trip incorrectly
• Metering vs protection CTs
• Accuracy classes including 0.5, 5P and PX
• Accuracy Limit Factor (ALF), composite error and saturation
• CT burden and why cable runs matter
• Knee point / excitation characteristics
• Toroidal, bar, bushing and split-core CTs
• Core balance and summation CTs
• Overdimensioned, anti-remanence and linear CTs
• Oil and SF6 insulated CTs
• CT analysers and what they actually test
• Secondary and primary injection testing
• Stability and operate shots for differential protection
• The potentially serious consequences of getting CT testing wrong
Current transformers are effectively the "eyes of the protection system". Without accurate CT measurements, protection relays can`t reliably determine what`s happening on the network or decide when a circuit breaker needs to trip.